1. Field of the Invention
This invention relates to removal of cores from near net shaped articles of manufacture such as castings and more particularly to removal of sacrificial cores by electrochemical machining.
2. Brief Description of the Prior Art
In the manufacture of articles having complex shapes it is desirable to minimize the amount of machining required to form the final shape. Similarly, in the manufacture of articles from materials that, are difficult to forge or machine, such as hard and refractory metals, machining steps are to be avoided, if possible. Accordingly, methods of manufacture have been developed in which the raw material is formed directly into a final shape, or at least a shape that requires little additional processing. Such manufacturing has come to be known as xe2x80x9cnear net shape formingxe2x80x9d.
For example, in the manufacture of a complex part from a refractory metal, it is sometimes necessary to provide one or more cavities, e.g., recesses and/or passages, within the body of the casting. Forming such cavities by conventional mechanical machining may be difficult, impractical, or even impossible for some refractory materials and/or some geometrical shapes of the cavity. Accordingly, processes have been developed to produce articles having such internal structures by near net processing methods such as investment casting, hot isostatic pressing (HIP), permanent mold casting, pressure casting or squeeze casting, injection molding, sintering, firing of a green preform, and the like. Typically, a sacrificial core is positioned within a mold in which the article is formed or within a preform that is subsequently formed to a near net final shape. For example, a core may be positioned within a casting mold, and the mold may then be filled with molten metal and cooled to solidify the metal.
After the article has been formed into its near net final shape it is removed from the shaping structure, e.g., a mold. However, ordinarily the core must be separately removed. In many cases the shape of the core or its close fit within the shaped article prevents it from being removed in one piece, especially in the case of precision castings or the like, where the final dimensions of the article are formed in the mold. Consequently, the use of sacrificial cores that are removed by destroying them has been developed.
Ceramic cores have been used in casting refractory metal parts. However, the removal of the hard ceramic must be accomplished by mechanical means or chemical leaching, both of which are time-consuming and costly. Accordingly, it has been proposed to use metal cores that can be removed by chemical leaching. For example, the use of steel cores has been proposed for casting articles from refractory alloys of cobalt, nickel, titanium and the like. The steel cores may be removed by chemical leaching with strong acids such as nitric and hydrochloric acids. However, even with the use of such corrosive materials, removal of the cores by chemical leaching is extremely slow and difficult, especially in deep recesses and narrow passages.
Accordingly, a need has continued to exist for a method of removing sacrificial cores from near net shaped articles of manufacture that is simpler and more rapid than methods hitherto employed.
The difficulties encountered in removing sacrificial cores from near net shaped articles of manufacture have now been alleviated by the method of the invention wherein the sacrificial core is made from a metal that can be electrolytically dissolved. The shaped article with sacrificial core embedded therein, together with a counterelectrode as a cathode, is immersed in or flooded with an electrolyte, and an electric current is passed between the core and the counterelectrode with the core being made generally anodic with respect to the counterelectrode, whereby the sacrificial core is electrochemically dissolved.
Accordingly, it is an object of the invention to provide an electrochemical method for removing a sacrificial core from a near net shaped article of manufacture.
A further object is to provide a method for removing a sacrificial core from a shaped article to provide a near net shaped cavity within the article.
A further object is to provide a method for electrolytically removing a sacrificial core from a casting.
A further object is to provide a sacrificial core for a near net shaped article of manufacture such as a casting.
A further object is to provide a sacrificial core having an integral electrode.
Further objects of the invention will become apparent from the description of the invention which follows.